The principal objective of this project is the development of a high-level computer diagnostic program in the broad field of internal medicine as an aid in the solution of complex and complicated diagnostic problems. To be effective, the program must be capable of multiple diagnoses (related or independent) in a given patient. A major achievement of this research undertaking has been the design of a program call INTERNIST, along with an extensive medical data base now encompassing almost 500 diseases and more that 3,000 manifestations of disease. Although this consultative program is designed primarily to aid skilled internists in complicated medical problems, the program may have spin-off as a diagnostic and triage aid to physicians assistants, rural health clinics, military medicine and space travel. Because INTERNIST is intended to serve a consulting role in medical diagnosis, it has been challenged with a wide variety of difficult clinical problems: cases published in the medical journals, cpc's, other interesting and unusual problems arising in the local teaching hospitals. In the great majority of these test cases, the problem-formation strategy of INTERNIST has proved to be effective in sorting out the pieces of the puzzle and coming to a correct diagnosis, involving in some cases as many as a dozen diagnostic entities. In this proposal, support is requested to continue development of the knowledge structures and heuristic procedures that constitute the INTERNIST model of medical expertise.